Geographic reuse of frequencies, sometimes called “frequency reuse,” is a distinguishing feature of cellular systems. Because a limited swath of the RF spectrum is available, frequencies are reused in order to provide a cellular system with an acceptable capacity. This is accomplished by breaking up the coverage area into a number of cells, or cell sites, each having a relatively low-powered transmitter. The transmitter has sufficient power to cover the cell site without interfering with other cell channels.
Frequencies can be reused by spacing “same channel” cells sufficiently far apart that the likelihood of interference is minimized. This permits two conversations to occur simultaneously on the same channel without conflict. Frequency reuse may be implemented using an architecture with a “reuse pattern.” A reuse pattern is a building block of cells that is repeated in order to build up to the coverage area.
For example, in an n=7 cellular system, seven cells define the reuse pattern. For example, there may be a first cell A1 surrounded by six cells B1–G1. Placed adjacent to these first seven cells is a second set of cells with the same pattern, an A2 cell surrounded by six cells B2–G2. Additional sets of seven cells are added (A3–G3, A4–G4, and so forth) to build up to the entire coverage area.
The spacing between same channel cells is a function of the reuse pattern. In an n=7 system, at least two cells have to be traversed to arrive at a same channel cell (e.g., the spacing between A1 and A2 or between C1 and C2). As n increases, spacing between same channel cells increases. As n decreases, spacing between same channel cells decreases.
Frequency reuse is one way to increase subscriber capacity in spite of the constraints imposed by the finite number of available channels. Frequency reuse has its limitations, however. For a given cell transmitter power and cell size, same channel cells can be spaced only so closely before interference becomes a problem.
Sometimes subscriber capacity can be increased through cell-splitting. In cell-splitting, each cell is split into subcells, with each subcell having its own transmitter. This permits greater frequency reuse, although the transmitter power of adjacent cells may have to be reduced to avoid interference. A significant drawback of cell-splitting is the cost of installing the new hardware. Additionally, many jurisdictions have regulations severely limiting cellular providers' ability to install new antennas.
Another disadvantage of cell-splitting is the increased complexity it creates in the cellular system. Furthermore, the likelihood of interference or dropped calls can increase. The burden on the Mobile Telephone Switching Office (MTSO), which coordinates these operations, thus increases significantly.
Another complication of cell-splitting is that as cell size decreases, the number and rate of “handoffs” increases. A handoff occurs when a cell phone moves from a first cell to a second cell. The MTSO controls handoffs, which entails commanding the cell phone to switch to a channel in the new cell. The connection between the cells and the MTSO is also switched so that the call is routed between the new cell and the MTSO. Thus, as the user moves between cells, the call is “handed off” from one cell to another.
Handoffs are necessary to manage moving callers in a cellular-type system. However, handoffs can create certain difficulties. The number and rate of handoffs can impose a significant processing burden on the MTSO. Because multiple cell sites may transmit during the handoff process, the overall noise floor of the RF environment is raised. This reduces overall capacity of the system.
When handoffs are mishandled, calls may be dropped or may be handed off to a less-than-optimum cell. If a call is handed off to a less-than-optimum cell, a handoff to a better cell may be required in a short period of time.
As wireless carriers continue to grapple with frequency reuse and handoff issues, new challenges have arisen on other fronts. In order to address routing of emergency calls placed by cellular phones, the Federal Communications Commission (FCC) has issued a series of orders. These orders mandate that wireless providers begin supplying location information to public safety answering points (“PSAPs”) in order to support an enhanced 911 (known as “E911”) capability for portable wireless devices (“PWDs”). According to the FCC mandate, wireless providers must provide a Phase I capability in 2000, followed by a more robust Phase II capability in 2001.
The Phase I capability requires that wireless carriers provide general location information. The general location information would locate the PWD to within a cell site or cell sector.
In Phase II, the wireless providers must provide specific location information. The Phase II localization accuracy requirement depends on whether the localization technique is network-based or handheld-based. For network-based localization solutions, the accuracy must be at least within 125 m at a one standard deviation probability (67%) and at least within 300 m at two standard deviations (95%). For handheld localization solutions, the accuracy must be at least within 50 m at one standard deviation and at least within 150 m at two standard deviations.
As a result of these FCC orders, a number of different techniques for localizing PWDs have been developed to support E911. Network-based solutions include TDOA (time difference of arrival), AOA (angle of arrival), TDOA/AOA in combination, and LPM (location pattern matching). Handheld-based solutions include GPS (global positioning system).
TDOA localization relies on the fact that a signal transmitted by a PWD can be received at multiple cell site transceivers at slightly different times. If the signal is received at three cell site transceivers, the differential timing information can be used to compute a position for the PWD.
AOA localization relies on the fact that a signal transmitted by a PWD is typically received at different angles at multiple cell site transceivers. Using direction-finding (compass) circuitry at the cell sites, the angle of arrival is computed at each cell site receiving the signal. By processing the angles of arrival, a position can be computed.
TDOA/AOA is a combined approach that relies on a synthesis of the TDOA and AOA techniques. In this approach, coordinate pairs may be computed for both the TDOA and AOA techniques. The two coordinate pairs may then be averaged or otherwise combined. On the other hand, this approach may provide for selecting one or the other technique in some circumstances.
The location pattern matching (LPM) technique has been proposed for use in urban environments where tall buildings and other obstructions cause signal reflection and multipath phenomena. In LPM, the signal of the PWD is received at multiple cell site transceivers. The acoustic component of the signal is then analyzed and compared to a database of signal characteristics. The processing and database comparison permit signal anomalies such as multipath and echoes to be used to localize the PWD.